Amid all that, the Capitals beat Thomas on a pair of third-period shots that he usually swats aside with ease and skated off with a 4-3 victory Saturday at TD Garden. Now the Bruins are facing a 3-2 series deficit and a do-or-die Game 6 Sunday at the Verizon Center, wondering whether they have the ammunition to force Game 7 Wednesday.

“We obviously don’t want to be here,’’ the Bruins’ Shawn Thornton said. “But we have been here before. The experience should help. We know we have to focus on one game, not the big picture.’’

On the winning power-play goal, Troy Brouwer took an in-stride pass from John Carlson. Brouwer had a step on Andrew Ference as he hurtled into the offensive zone. But Ference steered Brouwer wide right. Brouwer’s sharp-angle shot shouldn’t have troubled Thomas.

But with 1:27 remaining, Brouwer’s shot slipped past the Bruins’ most important player for the winning goal.

“He fooled me and beat me clean,’’ Thomas said. “He was coming down with a lot of speed. He shot. I read that the shot was going lower. By the time I even realized the shot was going that high, I didn’t even have time to raise my hand.’’

Earlier in the third, Thomas steered the rebound of a long-distance Joel Ward shot directly into danger. From the right-side wall, Ward flipped a puck on goal. Ideally, Thomas would have either smothered the shot or placed the puck into the right corner.

Thomas’s rebound control was off. With his right pad, Thomas booted the puck onto Mike Knuble’s stick. The ex-Bruin, having shrugged off Thornton’s backcheck, snapped a 2-2 tie by potting his first goal of the series at 3:21.

“It was to the far leg,’’ Thomas said. “It was on the ice. It was just slow enough that I couldn’t get the momentum to the puck to get the puck all the way into the far corner. I think I was going to put the rebound more in the middle like I do sometimes to get it to one of my guys.

“But I read that one of our guys was there. I didn’t want it to go off his shin pad and in. Unfortunately, it just happened to land right on the Washington guy’s stick.’’

Brouwer’s goal was the winner. But Knuble’s strike was a gut punch.

In the second period, the Bruins slipped into a two-goal ditch. As a stickless and injured Joe Corvo tried to hold his ground, Alexander Semin punched in the rebound of a Dennis Wideman shot at 11:16.

Just more than three minutes later, the Capitals grabbed a 2-0 lead. Thomas kicked out a Jason Chimera shot. David Krejci settled the rebound but sent a lazy clearing pass off the wall that Jay Beagle picked off. Beagle’s shot caromed off Johnny Boychuk’s blade and past Thomas.

But the Bruins halted Washington’s momentum with two goals before the break. Dennis Seidenberg scored at 17:21, one shift after Thornton belted Matt Hendricks to give his teammates some emotional lift.

Twenty-eight seconds later, the Bruins tied the game. Braden Holtby (34 saves) stopped a Boychuk shot. But Brad Marchand and Rich Peverley crashed the net to keep Holtby from covering the rebound. Marchand jammed in the puck at 17:49 to make it a 2-2 game.

The rink was rocking. The Bruins had some swagger in their skates.

“I thought those goals were big to get us back into the game,’’ Bruins coach Claude Julien said. “It’s unfortunate the clock was running out, because we were really picking up some steam.’’

Then early in the third, because of Knuble’s goal, the Bruins had to play catch up again.

“I’ll take full responsibility for that third goal,’’ Thornton said. “The rebound came out pretty hot. I tried to get my stick on top of his. But he got just enough of it. We’re both righthanded. We’re both swinging for it. He got it, so it’s my bad.’’

The Bruins regrouped after Knuble’s goal. At 8:47, they finally cracked their 0-for-14 power-play tumble. With Wideman in the box for cross-checking, Marchand started the play by driving with the puck down the right side. Marchand slammed on the brakes, then went back to Seidenberg at the right point.

Because of how hard Marchand had driven, the Capitals sagged back. After Seidenberg dished to the left point, Boychuk had enough time and space to rip a one-timer past Holtby to tie the game at 3.

The Bruins were in a good spot. But they fell down a man at 17:10 when Benoit Pouliot slashed Nicklas Backstrom on the left leg. Brouwer made the Bruins pay.

The Bruins were better in Game 6. After juggling his lines in practice Friday, Julien got some better performances from some of his players. Leading the pack was Marchand, scoreless through four games. Marchand practiced on the fourth line Friday, but played on the second line alongside Patrice Bergeron and Peverley. Marchand responded with an emotionally engaged 1-1-2 game.

Krejci and Milan Lucic, however, have yet to appear. They both recorded their first points of the series in Game 5 with helpers on Seidenberg’s goal. Tyler Seguin (0-0-0 in the series) had his best chance to score in the third, but Holtby robbed him with a skate save.

“We don’t have everybody going,’’ Julien said. “That’s the part we’re trying to do here - get some guys going. Some of those guys are important players for our hockey club. They’re not where they should be at this stage.’’

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